It is well known that various onium salts, upon exposure to radiation, are capable of forming a Bronsted acid, and that the Bronsted acid thus formed can cure a wide variety of materials. See, for example, UV Curing: Science and Technology, edited by S. Peter Pappas and published (1978) by Technology Marketing Corporation, 64 Westover Road, Stamford, Conn. 06902. The problem with such salts is that they do not absorb out to 400 nm, and commonly must be used in combination with a light-absorbing photosensitizer in order to carry out photoinitiation at longer wavelengths than 300 nm.
Research Disclosure Vol. 289, May 1988, page 298, published by Kenneth Mason Publications Ltd., London, England, describes sulfonium salts and oxysulfonium salts which, upon exposure to visible radiation, undergo irreversible intramolecular rearrangement to form a Bronsted acid. The light-absorbing capability of these sulfonium and oxysulfonium salts depends upon overlap of molecular orbitals, that is, .pi. resonance throughout the molecule. The photo products of these salts absorb at shorter wavelengths than the starting sulfonium and oxysulfonium salts.
European Patent Application 0 447 544, published Sep. 25, 1991, discloses onium salts which form Bronsted acids upon absorption of visible radiation by means of a chromophore joined to the remainder of the molecule by a linkage which interrupts .pi. resonance. The chromophore can be selected to match the wavelength of the visible light. The onium salts have; as a substituent on a S, Se, As, N, or P atom in addition to the linked chromophore; at least one electron withdrawing group which exhibits a lower unoccupied molecular orbital than the chromophore.
There is a need in the art for onium salts which absorb ultraviolet radiation, are highly efficient and are thermally stable to the release of acid. It is desirable that such salts be capable of forming a Bronsted acid upon exposure to ultraviolet light of a chromophore joined to the remainder of the molecule through an insulating linkage, since the chromophore could be selected to match the desired exposing radiation.